I'm not sure if this will be helpful with your situation, but here's what I went through: I am a first time chicken owner and none of my five girls would go up into the coop at night. I would go to the run every night and pick them up and put them inside their coop. After reading many threads on this website, I went to Home Depot and bought an inexpensive closet light that is magnetic. I put it inside the coop and turned it on at dusk and all of my girls "went into the light" and walked themselves into the coop. I used the lights for three nights and never needed them again. Not sure why your hen would decide suddenly to not go inside, but the light trick might help if nothing else did.
